ztest: replace `shell_fprintf(sh, SHELL_ERROR, ...)` with `shell_error`

Replace `shell_fprintf(sh, SHELL_ERROR, ...)` with
`shell_error(sh, ...)` since it brings the same underlying
action with less typing.

Note: `shell_error` already concatenates `\n` to format strings,
so we remove `\n` from the original code.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>